The invention relates to an aqueous and/or water-dilutable polyisocyanate crosslinker blocked with diisopropylamine, to a process of their preparation and coating compositions containing the crosslinker, especially one-component baking lacquers used as fillers in the automotive field.
The formulation of blocked polyisocyanate crosslinkers and optionally melamine-formaldehyde resins with, for example, hydroxyl-group-containing polyacrylates to so-called xe2x80x9cone-componentxe2x80x9d baking lacquers is known in the art. If the lacquers in question are aqueous lacquers, the mentioned individual components must be dispersible in water and stable to storage in the aqueous phase for at least 6 months.
There are only a few isocyanate-blocking agents that fulfil those requirements. Butanone oxime is one such blocking agent, but it requires baking conditions of at least 30 minutes/140xc2x0 C. in order to ensure crosslinking of the polyisocyanate blocked therewith. 3,5-Dimethylpyrazole is also a suitable blocking agent and leads to crosslinking of the polyisocyanate blocked therewith under baking conditions of only 30 minutes/130xc2x0 C., as is disclosed, for example, in patent applications WO 97/12924 and EP-A 0 942 023. The disadvantage of 3,5-dimethylpyrazole is that it is relatively complex to prepare. 3,5-Dimethylpyrazole is obtainable on an industrial scale by condensation of acetylacetone and hydrazine hydrate, which is complicated for example, due to the properties of hydrazine hydrate.
Diisopropylamine is a readily obtainable isocyanate-blocking agent. It is known from the patent literature that it is used in powder coatings and coil coating lacquers, see EP-A 0 096 210 and EP-A 0 900 814.
Surprisingly, it has been found that polyisocyanates blocked with diisopropylamine are stable to storage in the aqueous phase and do not, for example, continuously liberate CO2, as can be the case, for example, with polyisocyanates blocked with malonic acid diethyl ester or 1,2,4-triazole.
It is an object of the invention to provide storage-stable and readily obtainable aqueous or water-dilutable blocked polyisocyanate crosslinkers that have a lower crosslinking or baking temperature than polyisocyanate crosslinkers blocked with butanone oxime.
That object has been achieved with the blocked aqueous/water-dilutable polyisocyanate crosslinkers according to the invention.
The invention relates to an aqueous or water-dilutable blocked polyisocyanate crosslinker containing
a) 100 equivalent % of a polyisocyanate component,
b) from 50 to 85 equivalent % of diisopropylamine,
c) from 15 to 40 equivalent % of a monohydroxy- and/or dihydroxy-carboxylic acid as hydrophilizing agent, and optionally
d) from 0 to 15 equivalent % of a difunctional chain-extension component containing OH and/or NH2 groups,
wherein the equivalent ratio of NCO groups of component a) to isocyanate-reactive groups of components b), c) and d) is from 1:0.8 to 1:1.2
The invention also relates to a process for the preparation of the blocked aqueous and/or water-dilutable polyisocyanate crosslinkers, which process is characterised in that the polyisocyanate component a) and, optionally, 30 wt. %, based on the delivery form of the blocked polyisocyanate crosslinker, of a water-miscible solvent are placed in a vessel and reacted in a first reaction step at approximately 80xc2x0 C. with the monohydroxycarboxylic acid provided for the hydrophilization, the mixture is then cooled to approximately 50xc2x0 C., and the NCO groups that remain are blocked by addition of diisopropylamine.
The preparation of the blocked aqueous/water-dilutable polyisocyanate crosslinkers according to the invention can take place stepwise, for example as follows: the polyisocyanate component a) and, optionally, 30 wt. %, based on the delivery form of the crosslinker, of a water-miscible solvent, for example N-methylpyrrolidone, are placed in a vessel and reacted in a first reaction step at approximately 80xc2x0 C. with the monohydroxycarboxylic acid provided for the hydrophilization. The mixture is then cooled to approximately 50xc2x0 C., and the NCO groups that remain, approximately 80%, are reacted completely with diisopropylamine.
If hydrophilization is to be achieved using a dihydroxycarboxylic acid or a mixture of di- and mono-hydroxycarboxylic acid, it is possible, conversely, first to carry out the blocking reaction with diisopropylamine at approximately from 40 to 70xc2x0 C.; the reaction with the hydrophilizing component subsequently takes place with the residual NCO groups. The formation of crosslinked gel particles can be avoided in that manner.
After addition of the neutralizing tertiary amine, for example dimethylethanolamine, the carboxylic acid groups are converted into hydrophilic carboxylate anions. It is then possible, for example, by the addition of further water-miscible solvent, to prepare a water-dilutable blocked polyisocyanate crosslinker having a solids content of approximately 65% and a viscosity (at 23xc2x0 C.) of approximately 10,000 mPas or, by the addition of water, to prepare a water-dispersed blocked crosslinker having a solids content of approximately 30% and a viscosity (at 23xc2x0 C.) of approximately 400 mPas.
The invention relates also to the combination of the aqueous and/or water-dilutable blocked polyisocyanate crosslinkers with water-soluble and/or dispersible polyhydroxyl compounds in the preparation of aqueous baking lacquers optionally containing conventional auxiliary substances and additives of coating technology. These combinations have advantages in automotive lacquering.
Polyisocyanates a) can be any known lacquer polyisocyanates having aliphatically and/or cycloaliphatically bonded isocyanate groups and an isocyanate content of from 7 to 30 wt. %, preferably from 12 to 25 wt. %. Preferred lacquer polyisocyanates include those that are based on 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (IPDI) and/or bis-(4-isocyanatohexyl)-methane (H12MDI) and that contain biuret, isocyanurate, aminooxadiazinedione (asymmetric trimer), urethane and/or uretdione groups. It is also possible to use any desired prepolymers of those polyisocyanates with various polyols, such as polyesters, polyethers, polyacrylates or mixtures thereof.
It is also possible to use polyisocyanates or short-chain NCO prepolymers based on aromatic diisocyanates, such as, for example, diisocyanatotoluene (TDI) or 4,4-diisocyanatodiphenylmethane (MDI).
Polyisocyanates based on HDI or H12MDI and containing isocyanurate groups are more preferred as component a).
Diisopropylamine is suitable according to the invention as the blocking agent from the group of the secondary amines. It is substituted or deblocked under the baking conditions of 130xc2x0 C./30 minutes that are required industrially and, like butanone oxime or 3,5-dimethylpyrazole, blocks isocyanate groups in aqueous lacquers in a stable manner, without the liberation of CO2 as a result of hydrolysis.
Suitable hydrophilizing components c) include preferably monohydroxy- and dihydroxy-carboxylic acids that carry one or two methyl groups in the (xcex1-position. Particular preference is given to hydroxypivalic acid (hydroxymethyldimethylacetic acid) and mixtures of hydroxypivalic acid with dimethylolpropionic acid.
Suitable difunctional chain-extension components d) include diamines, diols and also hydroxyamines having a molecular weight in the range from 32 to 300. Examples include hydrazine, ethylenediamine, isophoronediamine, the bisketimine of isophoronediamine and methyl isobutyl ketone, 1,4-dihydroxybutane, ethanolamine, N-methylethanolamine, hydroxyethylethylenediamine, the adduct of 2 mol of propylene carbonate and 1 mol of hydrazine of formula (I) 
Suitable neutralizing agents include dimethylethanolamine, methyl-diethanolamine, triethylamine, N-methylmorpholine or 2-amino-2-methylpropanol.
Suitable stabilizers to protect the crosslinkers against thermal yellowing include hydrazines and HALS amines (Hindered Amine Light Stabilizers) such as Tinuvin 770 DF (trade product of Ciba Corporation). Additives and further auxiliary substances can also be added.
The crosslinkers according to the invention can be used in a dispersed form either up to 80% by weight in a water-dilutable solvent or up to 50% by weight predominantly in water. Suitable water-dilutable solvents include butyl diglycol acetate, butyl diglycol, N-methylpyrrolidone or any desired mixtures thereof.
The polyisocyanate crosslinkers according to the invention have the following advantages:
The crosslinkers and the aqueous baking lacquers formulated therewith are stable to storage for at least 3 months.
The diisopropylamine blocking agent is an inexpensive, large-scale chemical.
The baking temperature of the aqueous lacquers is 30 minutes/130xc2x0 C., and is therefore 10xc2x0 C. lower than that of a crosslinker blocked with butanone oxime.
The simple composition and the good anticorrosive action (salt spray test) are demonstrated by means of the Examples which follow.